1. Field of the Invention
The present invention relates to a sensor module for inkjet printing devices having one or more cartridges for color operation.
2. Description of the Related Art
An ink jet printing mechanism uses cartridges, often called xe2x80x9cpensxe2x80x9d, which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cinkxe2x80x99, onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. The ink jet printing mechanisms may be used in different devices such as printers, plotters and the like. For the sake of convenience, in what follows reference will be made only to large format ink jet printers or plotters to illustrate the concepts of the present invention.
Different nozzles are used for different colors. Ink jet printers usually print within a range of 180 to 2400 or more dots per inch. The ink drops are dried upon the printing support soon after being deposited to form the desired printed images.
There are several types of ink jet printheads including, for example, thermal printheads and piezoelectric printheads. By way of example, in a thermal ink jet printhead the ink drops are ejected from individual nozzles by localized heating. Each of the nozzles has a small heating element. An electric current is made to pass through the element to heat it. This causes a tiny volume of ink to be heated and vaporized instantaneously by the heating element. Upon being vaporized, the ink is ejected through the nozzle. An exciter circuit is connected to individual heating elements to supply the energy impulses and, in this manner, to deposit in a controlled way ink drops from associated individual nozzles. These exciter circuits respond to character generators and other imaging circuits to activate selected nozzles of the printhead in order to form the desired images on the printing support.
Thermal inkjet printing is based on accurate ballistic delivery of small ink droplets to exact locations onto the paper or other media. One key factor for sharp and high quality images stems from the accuracy of the droplet placement. Droplet placement inaccuracy results in fact in line discontinuity and roughness, as well as banding and color inconsistencies.
Droplet placement inaccuracies are caused by imperfections and variations of the mechanical and geometrical characteristics of the printer and printhead, as well as their functional performances. The defects caused by droplet placement errors appear in a variety of ways and may depend on the print modes being used (i.e. the sweep velocity of the printhead over the paper and the direction of printing).
Full color printing and plotting requires techniques for correcting different causes of droplet placement inaccuracies. Some of these techniques, using a sensor module for measuring printing errors in appropriate printed patterns, are disclosed in EP 0 622 237.
The sensor module described in said patent includes two light emitting diodes (LEDs), two lenses, a phase plate of an opaque material with transparent squared openings and a photodetector. As the sensor module scans the test pattern, an output signal is provided and examined by an appropriate circuitry to measure the deviation of the printed test pattern with respect to nominal references.
The electrical signal produced by the photodetector is amplified with a fixed gain and the result is sampled with an analog-to-digital converter (ADC). The resulting set of samples from the ADC provides the information required to correct the cause of the inaccuracy by means of techniques of adjusting the time at which a droplet is being ejected.
Using this sensor module, two main problems arise. Firstly, its calibration has only one parameter to play with: the light produced by the LEDs. If it is too large, the resulting electrical signal saturates the ADC and the computed sequence of samples does not reflect the real surface previously scanned. If it does not reach the necessary value, the noise acquired together with the signal render the system totally blind. The calibrated level of light is therefore obtained at the level in which the samples start to saturate the ADC. This calibration method does not ensure the use of the best signal available, in terms of signal to-noise-ratio; therefore failure tolerance to noise is dramatically decreased, specially when LEDs provide very different levels of intensity.
Secondly, since the LEDs and photodetector are tied to the gain of the amplifier and the ADC dynamic margin, changing the optical components of the sensor modules implies modifying the module circuitry.
There is a need for solving these problems in order to improve the performance of current and new correction systems based on the measurement of droplet placement inaccuracies in printed patterns.
The present invention provides a selfcalibrated sensor module for inkjet printing devices.
According to the present invention, the sensor module includes a new circuitry and a new calibration method to provide the best output signal independently of optical component functionality variations and external light source influences.
The circuitry for processing the photodetector output signal is designed to process that signal by a bank of amplifiers or by an amplifier of variable gain and includes an input for adding an offset to the signal.
The calibration system, implemented in a processing unit, calibrates the sensor module, firstly finding the level of light that should be applied to the LEDs to maximize it so as to grant the best signal possible, in terms of the signal to noise ratio (SNR), Secondly, it determines which amplification factor will be used in order to ensure that the resulting sampled signal is not saturated. Thirdly, it determines the necessary offset to be added to the signal to center it in the dynamic margin of the ADCs.